One year of downwelling spectral radiance measurements from 100 to 1400 cm⁻¹ at Dome Concordia: Results in clear conditions

Abstract

The present work examines downwelling radiance spectra measured at the ground during 2013 by a Far Infrared Fourier Transform Spectrometer at Dome C, Antarctica. A tropospheric backscatter and depolarization lidar is also deployed at same site, and a radiosonde system is routinely operative. The measurements allow characterization of the water vapor and clouds infrared properties in Antarctica under all sky conditions. In this paper we specifically discuss cloud detection and the analysis in clear sky condition, required for the discussion of the results obtained in cloudy conditions. First, the paper discusses the procedures adopted for the quality control of spectra acquired automatically. Then it describes the classification procedure used to discriminate spectra measured in clear sky from cloudy conditions. Finally a selection is performed and 66 clear cases, spanning the whole year, are compared to simulations. The computation of layer molecular optical depth is performed with line‐by‐line techniques and a convolution to simulate the Radiation Explorer in the Far InfraRed‐Prototype for Applications and Development (REFIR‐PAD) measurements; the downwelling radiance for selected clear cases is computed with a state‐of‐the‐art adding‐doubling code. The mean difference over all selected cases between simulated and measured radiance is within experimental error for all the selected microwindows except for the negative residuals found for all microwindows in the range 200 to 400 cm−1, with largest values around 295.1 cm−1. The paper discusses possible reasons for the discrepancy and identifies the incorrect magnitude of the water vapor total absorption coefficient as the cause of such large negative radiance bias below 400 cm−1.

Antarctic Ice Cloud Identification and Properties Using Downwelling Spectral Radiance From 100 to 1,400 cm−1

“…The selected windows are, respectively, representatives of a moderate high and a moderate low absorption of ice in the FIR band. In both channels, residuals computed in clear conditions (Rizzi et al, 2016) show a slightly negative bias (not shown here). At 295 cm −1 , the residuals show a large variability, and the average is close to zero; no trend is observed in the plot for increasing COD.…”

“…For each case, one REFIR‐PAD spectrum (i.e., the average of four consecutive acquisitions lasting 5.6 min) is used unless the scene is particularly stable. In this latter case, two or three spectra are averaged (using the same procedure adopted for clear sky; Rizzi et al, ) depending on the stability of observed scene.…”

“…For each case, one REFIR-PAD spectrum (i.e., the average of four consecutive acquisitions lasting 5.6 min) is used unless the scene is particularly stable. In this latter case, two or three spectra are averaged (using the same procedure adopted for clear sky; Rizzi et al, 2016) sity of conditions analyzed during the campaign. The case in Figure 4a is representative of a thin high cloud that does not lead to any precipitation events in the following hours.…”

Estimate of Radiosonde Dry Bias From Far‐Infrared Measurements on the Antarctic Plateau

“…Black circles are the weighted averaged residuals between simulated and measured radiances δL wt (ν w ) computed using all cases contained in the SCS data set and vertical bars are one standard deviation around the yearly mean σ Lwt (ν w ); the red dashed lines connect the values of the total spectral measurement uncertainty in each microwindow (σ pwa (ν w )). The green line with diamonds are the corresponding residuals reported in Rizzi et al (2016), and the vertical green lines are the one standard deviation around the yearly mean. The error bars are slightly displaced for visual purposes.…”

“…The present paper analyzes data from the "Proprietá Radiative del vapore acqueo e delle Nubi in Antartide" (radiative properties of water vapor and clouds in Antarctica) research project, whose research activities started in December 2011 with the Summer Campaign of the XXVII scientific expedition and are supported by the Italian "Programma Nazionale di Ricerche in Antartide" (Palchetti et al, 2015). Rizzi et al (2016) (hereafter called CR1) analyze a full year of downwelling radiance spectra, measured during 2013 by a ground-based Fourier transform spectrometer (FTS), called Radiation Explorer in the Far InfraRed (REFIR)-Prototype for Applications and Development (PAD). Of relevance for the present discussion is the comparison of the radiances measured in 66 clear-sky days (collectively named Selected Clear Set or SCS) to simulations obtained with a state of the art methodology: the monochromatic molecular optical depth (σ m ) in each layer is computed using the Line-by-Line Radiative Transfer Model (LBLRTM) (Clough et al, 2005) version 12.2, using the Atmospheric and Environmental Research (AER) line parameter database v.3.2 and the Mlawer-Tobin-Clough-Kneizys-Davies (MT-CKD) version 2.5.2 water vapor continuum model (Mlawer et al, 2012).…”